1. Field of the Invention
The present invention is directed to the decontamination of industrial fluids and, in particular, to the decontamination of fluids utilized in electrocoating processes through low power, high frequency, ultrasonic radiation.
2. Description of the Related Art
Electrocoating (e-coating) generally relates to a coating method in which an electrical current is utilized to deposit a coat to a object. As used herein, preferred embodiments often describe “e-coating” as a painting method, but the term “e-coating” is broad enough to cover any suitable coating methods.
Electrocoating works on the principle that oppositely charged particles attract each other. More specifically, an electrocoating system typically applies a DC charge to a metal part (or any part desired to be painted) immersed in a bath of oppositely charged paint particles. The paint particles are drawn to the metal part, and paint is deposited on the part, generally forming an even, continuous film over the surface, including crevices and corners, until the coating reaches the desired thickness. After the desired thickness is achieved, the part can be insulated, to stop the deposition of the paint particles by stopping the attraction.
A typical electrocoating system consists of a number of components that can help maintain line parameters. For example a rectifier usually supplies the DC electrical charge to the bath, so to enable coating of the immersed object. In addition, circulation pumps often maintain proper paint mix uniformity throughout the electrocoat bath. Furthermore, temperature control of the paint bath is typically provided by a heat exchanger and/or chiller. Electrocoating systems often employ tank filters to remove dirt particles that are introduced into the paint system. Typically, ultrafilters are used to control paint conductivity, produce permeate for rinsing, and allow for recovery of paint solids.
E-coating generally consists of numerous steps including: electrodeposition pre-treatment, electrodeposition, painting, bathing, rinsing, and post rinsing, each of which involves industrial fluids. Before e-coating, the metal pieces are generally treated in a phosphatizing process, and then rinsed.
Unfortunately, fluids utilized in electrocoating processes, especially water-based fluids, are susceptible to bacteria, algae, fungi, yeasts, molds and other microbial propagation. The charged media encountered in e-coat installations are prone to bacterial developments, due to the high surface/volume ratios of the solid particles present in the formulations as well as their high organic content. Biological contamination of these fluids can be costly and dangerous, thus, some biological control for these fluids is desired.
Industrial fluids utilized in electrocoating processes can include complex compositions, slurries, and emulsions, as well as neat or filtered liquid. The liquid vehicle for these compositions is often demineralized or deionized water (DI) (See U.S. Pat. No. 5,393,390, to Freese, et al.). Coating compositions often contain various types of ingredients. For example, electrodeposition lacquers are often multicomponent aqueous emulsions or dispersions. Thus, it is advantageous to protect the formulations as well as the liquid medium itself.
In e-coating, one of the most abundant bacteria is the Burkholderia Cepacia which is a gram-negative bacterium. Human infection can be caused by B. cepacia, especially in patients with cystic fibrosis and chronic granulomatous disease, and can often be fatal.
It is important to note that biological fouling usually affects the entire e-coating system, including the circuitry, the filtration devices, as well as the coatings. Biological contamination of these fluids can also diminish the quality of the applied finish on parts, and increase both down time and maintenance costs. Biological fouling can also be deleterious for the quality of the finished product
Biological contamination is usually associated with the formation of biofilm. Utilizing conventional treatments, it was often not possible to significantly reduce biofilm, thus, there is still a need for an effective biofilm removal from the circuit equipment and pipes. A number of patents, such as U.S. Pat. Nos. 5,971,757, 5,961,326, 5,749,726, and 5,204,004 teach the use of a variety of replaceable in-line water filters for trapping bacteria, such as biofilm sloughing.
To minimize these risks, hazards, and other negative effects of contaminated fluids utilized in electrocoating processes, many facilities add appreciable levels of various biocides to fluids utilized in electrocoating processes, to kill and inhibit the growth of microorganisms. In practice however, these agents are of limited usefulness. In addition to costing more money, there are limits on the amount of biocide which can be incorporated into an e-coating fluid without compromising the effectiveness of the fluid. Furthermore, these conventional techniques do not provide long term reduction of microbial counts in large industrial systems.
To obtain sustained and long usage of the electrocoating fluid, it is desirable that the treatment of the electrocoating fluid does not modify the electrocoating fluid or emulsion in its desired composition or characteristics. A major problem with biocides is that they can be detrimental to the efficacy and integrity of the e-coating fluid. Ultimately, the microorganisms overcome the biocides and the microbial degradation of electrocoating fluid and contaminants results in foul odors in the work environment.
In addition to using biocides, other facilities have used the following methods to treat e-coating fluids: the use of radioactive metals (e.g., U.S. Pat. No. 5,011,708 to Kelly, et al.), biofilm removal strategies (e.g., U.S. Pat. No. 6,183,649 to Fontana, and U.S. Pat. No. 5,411,666 to Hollis, et al.), physical methods, such as electrolysis (See U.S. Pat. No. 6,117,285 to Welch, et al. and U.S. Pat. No. 5,507,932 to Robinson), galvanic cell treatments (See U.S. Pat. No. 6,287,450 to Hradil, and U.S. Pat. No. 6,746,580 to Andrews, et al.), and pulsed light sterilization (See U.S. Pat. No. 6,566,659 to Clark, et al.).
Previous treatment methods have also used biocides to treat industrial installations used in e-coating. These installations often use filtering systems for the transfer and recirculation of fluids charged with clogged paint and coarse solid particles etc. Biological contamination of these filters was treated by the use of biocides. In situ cleaning systems (See U.S. Pat. No. 5,403,479 to Smith, et al.) of fouled microfiltration (MF) or ultrafiltration (UF) using semi-permeable hollow fiber membranes has also been used when flux decreased to an unacceptably low level.
Thus, conventional methods for the decontamination of e-coat fluids include membrane filtration to remove microorganisms, and/or the addition of chemicals, or other additives to kill and/or inhibit proliferating microorganisms in the fluid.
It is important to note that the liquid involved in e-coating processes is often mainly water. Thus, an industrial plant often needs to treat large amounts of demineralized and/or deionized water. There is typically a continuous replacement of spent water, due to evaporation, spillage, and drift. As contaminated deionized water is very corrosive, and the addition of anticorrosion chemicals is not always estimated as the best solution, there is still a need to cheaply and safely treat this deionized water, without significantly diminishing the effectiveness of the fluid.
While the use of high power, low frequency ultrasound has been proposed to treat surfaces locally for keeping them free of scaling, fouling and dirt (See U.S. Pat. No. 5,386,397, to Urroz) there is still a need in the art to decontaminate fluids used in the e-coating processes.
It is also important to note that high solid content in fluids is usually detrimental for chemical, UV, or low frequency ultrasonic mechanical treatments. More specifically, the solids often act as sorbents or shields to the transmission of the irradiation. Thus, the opacity and heterogeneity of the medium is often a hindering factor for its decontamination under classical methods. For example, opacity of the medium is specifically detrimental to UV treatment.
Accordingly, there is a need in the art for an effective and new method of treating fluids utilized in electrocoating processes without the use of large amounts of biocides, and which can provide uniform protection, or substantially uniform protection with time.